Such watches are well known. They generally comprise, behind the dial, a disc bearing indications of the days of the week, and another disc on which are inscribed numbers 1 to 31 for the ordinals of the month. The two discs are driven by the watch's movement to display the indications designating the current ordinal of the month and the corresponding day of the week through an aperture that is formed in the dial.
Besides this information about the current day, it is often most useful to have the same information about a future day, for instance for the purpose of making an appointment. Present analog electronic watches, however, do not provide such an indication, which, on the other hand, is given by some mechanical calendar watches.
These mechanical watches have been known for a long time and comprise a stationary, ring-like graduation which is disposed on the dial and which is centered on the axis of the time-indicating hands. This graduation is divided into 35 equal parts and each part bears the indication of one day of the week for 5 consecutive weeks. Another graduation, which is also divided into 35 parts and which is both concentric with the preceding graduation and able to rotate about its center in response to manual actuation, has inscribed thereon numbers 1 to 31. Each number corresponds to an ordinal of the month and is inscribed, opposite a day of the week, in one of the 31 consecutive parts of the graduation which thus provides a free space extending over 4 days. These watches further comprise an indicator hand, or index, which is driven by the movement so as to travel 1/35th of a revolution in 24 hours around the axis of rotation of the other hands.
In these conditions, it suffices to place, at a given moment, the indicator hand on the correct day and move the graduation bearing the numbers to the position in which the corresponding day of the month is brought into alignment with this hand whereby the watch may continue to indicate indefinitely the days of the week, but only till the end of the current month, and the ordinals of the month. At the beginning of each month the graduation bearing the ordinals must then be moved, e.g., by 4 days if the past month was January, whereby the indicator hand may show the first of February.
Since both graduations are fully visible, it is possible in these watches to permantly read off the correspondence, for the current month, between the ordinals of the month and the days of the week.
A mechanical watch of this type is described in detail for instance in Swiss Specification No. 332899. In this particular form of construction, the graduation bearing the days of the week is on the dial while that indicating the ordinals of the month is disposed on a rotary glass. At the beginning of each month the glass must therefore be placed manually in the correct position. This operation of course amounts to a constraint that is incompatible with current trends towards simplified watch controls, in particular with electronic watches.